How to redirect to FTP server in LAN?

I have 2 servers Ubuntu. One is inet provider with static public IP adress (ext. 80.122.45.122, int 192.168.1.50). And second is FTP server (and backup server etc.), in the same LAN, with IP static 198.168.1.51. How to redirect to my FTP server from outside of LAN ? FTP server is 192.168.1.51, port 21. Thanks.